1. Field of the Invention
The invention relates to the field of electric motors, and specifically to the field of piezoelectric motors.
2. Background
In general, miniature electric motors are utilized in the fields of microsurgery, microbiology, and microelectronics, to name a few. Unfortunately, the miniaturization of electric motors for use in these and other fields is typically associated with a sharp decrease in motor efficiency. For example, as miniature electric motors are reduced to operate using diameters of less than 10 mm, it is not unusual to see efficiency degradation to as low as 2% or less. Even though this effect does not generally apply to the miniaturization of the piezoelectric motors, miniaturization itself can still lead to design complications. For example, miniaturized piezoelectric motors are generally prone to having poor reliability and experiencing degradation of motor parameters such as resolution, torque, and speed, to name a few.
In general, the conventional piezoelectric motor designs incorporate a piezoelectric resonator with electrodes to stimulate the simultaneous excitation of longitudinal and bending vibrations. The piezoresonator interacts with a moving rotor by friction over a contact area between the resonator and rotor. In such motors, a flat piezoelectric resonator is generally used, in which both longitudinal and flexural deformations are excited.
In some conventional piezoelectric motor design, a drive mechanism can be provided which uses a rectangular piezoresonator with electrodes positioned to excite both longitudinal and lateral vibrations. Consequently, the piezoresonator can interact by contacting a moving part (rotor) through at least one contact pad. In general, the size of the resonator and the shape of the resonator electrodes are constructed to enable the simultaneous formation of the first longitudinal mode and the second bending mode of vibration along the length of the resonator. If the frequencies of these two modes are equal, the contact pad typically vibrates. In particular, the contact pad follows an elliptical path a plane parallel to the plane containing all electrodes. Via friction, the contact pad causes rotation of the rotor. Such piezoresonators are typically polarized normal to the plane containing the electrodes with a common electrode occupying one flat surface of the piezoresonator. The opposite flat surface is generally divided into four symmetrically disposed sectors each one of which has an active electrode.